Fuel cells are well known and are typically arranged in a fuel cell stack in cooperation with support systems to form a fuel cell power plant. Such plants are used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. For fuel cells of the prior art to remain in water balance, it is known that an amount of water leaving the fuel cell must not be greater than an amount of water generated by the fuel cell during operation. Additionally, water must move into, move through, and diffuse and/or evaporate out of porous layers of the fuel cell at a stable rate in order to humidify dry reactant streams, to prevent drying out of a proton exchange membrane (“PEM”) electrolyte, to remove heat generated by the fuel cells, and to perform other related tasks. Such water balance requirements are especially difficult to accomplish for fuel cells used in transportation vehicles wherein ambient temperatures vary from well-below the freezing temperature of water to over forty degrees centigrade (“40° C.”)
For example, when ambient air is hot and dry and is the oxygen containing oxidant stream, as the air enters the fuel cell at a very low relative humidity, water is readily evaporated into the oxidant stream from a fuel cell porous layer that defines an oxidant flow field, from other known porous support layers, and from any PEM electrolyte, etc. As the oxidant stream moves through the fuel cell, it is heated and additional fuel cell product water is also evaporated into the oxidant stream until it is saturated. When the oxidant stream nears a fuel cell oxidant outlet, water condenses out of the stream back into the porous fuel cell support layers. As disclosed in U.S. Pat. No. 6,322,915 that issued on Nov. 27, 2001 and in U.S. Pat. No. 6,521,367 that issued on Feb. 18, 2003, (both Patents being owned by the owner of all rights in the present invention), considerable effort has been undertaken to maintain satisfactory water balance in such fuel cells by use of “electrolyte dry-out barriers” as well as by actively circulating a coolant stream adjacent reactant stream inlets and outlets and within coolant channels defined in the porous fuel cell support layers to enhance proper reactant stream humidity, heat removal, and transport of condensed fuel cell water.
Such solutions, however, involve considerable parasitic power to actively circulate a coolant stream; involve care in minimizing freeze-related problems of the coolant stream and fuel cell water; impose enhanced weight and volume requirements for the fuel cell power plant; and, require elaborate manufacturing effort and cost. Consequently, there is a need for a fuel cell that provides for passive water balance to minimize or eliminate active coolant circulating and related water management systems.